/*
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Follow up:
Can you solve it without using extra space?
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *slow = head;
        ListNode *fast = head;
        while (fast) {
            slow = slow->next;
            fast = fast->next;
            if (fast) fast = fast->next;
            if (slow && slow == fast) break;
        }
        if (fast) {
            slow = head;
            while (slow != fast) {slow = slow->next; fast = fast->next;}
        }
        return fast; 
    }
};
